The present invention relates to machines for grinding articles, and in particular to a machine for grinding the edges of ophthalmic lenses to predetermined outlines.
In manufacturing ophthalmic lenses for eyeglass frames, a lens blank is first surface ground and polished to a predetermined prescription. The resulting lens has a circular periphery or edge, and is of a sufficient diameter that it may be ground to an outline corresponding to that of an eyeglass frame in which it is to be mounted and provided with a bevel for mounting in the frame.
Machines for grinding the peripheries of lenses to predetermined outlines and for forming bevels on the peripheries are referred to as bevel edging machines or bevel edgers. Such machines conventionally include a rotatable workholder for supporting and rotating a lens and for bringing the periphery of the rotating lens against either a rough grinding wheel or a finishing or beveling wheel having a V-shaped groove in its surface for grinding a bevel on the lens periphery. A pattern having an outline corresponding to that to which the lens is to be ground is carried on an end of the workholder, and controls movement of the workholder toward and away from the rough grinding and beveling wheels to cause the periphery of the lens to be ground to the configuration of the pattern. During rough grinding, the pattern is held above a first clapper switch or roughing wear plate until the lens periphery has been ground by an amount permitting the pattern to move against and actuate the switch, whereupon the workholder is rotated to bring new unground portions of the lens periphery into engagement with the grinding wheel, which moves the pattern away from the switch to stop rotation of the workholder until a sufficient amount of the new portion of the periphery has been ground away, whereupon the cycle is repeated. The peripheral portions of the lens are thus sequentially engaged with the grinding wheel to rough grind the lens edge to a configuration corresponding to that of the pattern, and in the usual roughing cycle rough grinding of the lens is limited to two complete revolutions of the workholder, after which it is assumed that the lens has the selected configuration.
To form a bevel on the lens edge, after rough grinding the lens is positioned opposite from the finishing wheel and the pattern above a second clapper switch or finishing wear plate, which switch also controls rotation of the workholder in accordance with engagement of the pattern therewith. The beveling cycle then proceeds in a manner similar to that of the roughing cycle, with the lens being moved toward and away from the bevel edging wheel by engagement of the pattern with the second switch and with the switch controlling rotation of the workholder upon the pattern moving against it. As for the roughing cycle, the bevel edging cycle is usually limited to two complete revolutions of the workholder.
For a more complete description of a known type of bevel edging machine, attention is invited to Stern U.S. Pat. No. 3,332,172, issued July 25, 1967 and assigned to the assignee of the present invention, the teachings of which are incorporated herein by reference.
Although relatively thin lense of small minus powers may often be rough ground and bevel edged to the proper size within two rotations of the lens during each grinding cycle, it often happens that thicker lenses of larger minus powers require more than two revolutions during one or both cycles to be fully ground to the selected configuration, which is not accommodated by the two rotation limit of conventional bevel edgers. This gives rise to a further disadvantage since if the lens periphery is not fully ground during the revolution limited grinding cycles, that circumstance is usually not ascertained until after the lens has been removed from the machine and an attempt is made to fit it into an eyeglass frame. If further grinding of the lens is required, it is then difficult to properly align the bevel already formed on the lens with the V-shaped groove in the finishing wheel so that another finishing cycle can be performed, which can result in an imperfect bevel and fit of the lens in the eyeglass frame. Also, most conventional bevel edgers do not readily accommodate automatic formation of bevels toward the front faces of lenses having different base curves, but instead require visual observation and manual effort by an operator to form the bevel at that position so that the lens will have a cosmetically acceptable appearance in the frame.
Another disadvantage of conventional bevel edgers is that the workholder is elongate an supported for rotation by a carriage, the pattern is at an end of the workholder and the carriage gravity urges the lens against the grinding wheels and the pattern against the clapper switches. In consequence, as the lens is ground the pattern supports the weight of the carriage and a bending moment of force is exerted on the workholder, which deforms the workholder and causes an inaccuracy between movement of the lens and the pattern. As a result, the lens periphery is often not ground to a configuration precisely corresponding to that of the pattern.